The invention relates to a means of anchorage of anode pins or stubs in a carbon anode for fused salt electrolytic cells for producing aluminum, comprising a casting which is attached to the lower part of the anode pin or stub and made up of cast iron which is cast in and solidified in at least partly undercut holes in the side of the anode pins or stubs.
In the electrolytic production of aluminum from aluminum oxide, the latter is dissolved in a fluoride melt consisting for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum or a solid body which can be wet by aluminum forming the cathode. Dipping into the electrolyte from above and secured to an anode beam are anodes which in conventional reduction processes are made of amorphous carbon. As a result of the electrolytic decompositon of the aluminum oxide oxygen is formed at the anodes; this oxygen combines with the carbon of the anode to form CO.sub.2 and CO. The carbon anodes must therefore be replaced by new anodes at periodic intervals.
The electrolytic process takes place in general in a temperature range of 940.degree.-970.degree. C.
The several hundred kilogram heavy prebaked carbon anodes are attached to the lower end of the anode rod i.e. the anode pins or stubs. Anode pins are round in cross section, anode stubs on the other hand quadratic or rectangular. For attachment to the anode rods, these are inserted in holes in the carbon block suitably shaped to accept the anode pins or stubs. The space surrounding these is filled with a rammed mass of carbon which can be baked to form a coke, or filled with cast iron. The material used for attaching these parts must exhibit not only high mechanical stability, but also good electrical conductivity at the contact faces.
As the carbon anodes employed represent a significant cost factor in the electrolytic production of aluminum, the depth of the holes for the anode pins or stubs is important. A spent anode must be changed before the naked iron pins or stubs come into contact with the molten electrolyte.
The German Pat. No. DE-PS 1 937 411 shows how the depth of the holes for the pins or stubs can amount to only 3-6 cm, in particular thanks to the use of undercutting of the holes.
Although Pat. No. DE-PS 1 937 411 results in a significant improvement with respect to depth of contact means and thus the amount of anode wastage, a further problem exists which cannot be adequately solved in accordance with this patent. On casting in the cast iron the anode rods stand on the bottom of the holes for the pins or stubs, they are surrounded and thus clad with cast iron in the lowest part. As the steel of the pins or stubs does not have the same coefficient of thermal expansion as the cast iron, on cooling, the cast iron which has the larger coefficient of expansion shrinks more than the steel. As a result, when the anode is put into service in the cell, there is a gap between the bottom of the hole for the pin or stub and the lower face of the pin or stub. This has the effect of causing most of the direct electric current to flow into the sides of the anode during the reduction process, and consequently a higher voltage drop results.